The general aim of the proposed study is to gain an understanding of the contractile mechanism in vertebrate smooth muscle fibers by investigating some fundamental structural properties of the contractile apparatus. The objectives are 1) to futher refine a purification procedure and extend the characterization of the composition and substructure of the 100 Angstrom Unit-diameter filaments, 2) to examine the three dimensional organization into the cytoskeleton within intact smooth muscle fibers by conventional and high voltage electron microscopy, and 3) to determine some properties of the subunits of smooth muscle myosin.